Growth factors, growth factor response elements, and the cardiac phenotype

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Abstract
Fibroblast growth factors (FGF) and type β-1 transforming growth factor (TGFβ1) are pleiotropic regulatory peptides which are expressed in myocardium in a precise developmental and spatial program and are up-regulated, in the adult heart, by ischemia or a hemodynamic burden. The accumulation of trophic factors after aortic banding supports the hypothesis that autocrine or paracrine pathways might function to mediate, in part, the consequences of mechanical load. Our laboratory has demonstrated that cardiac muscle cells are targets for the action of peptide growth factors and, more specifically, that modulation of the cardiac phenotype by basic FGF (bFGF) and TGFβ 1 strongly resembles the induction of fetal cardiac genes — including skeletal α-actin (SkA), β-myosin heavy chain, and atrial natriuretic factor — which are characteristic of pressure-overload hypertrophy. Unexpectedly, and despite effects like those of bFGF on five other cardiac genes, acidic FGF (aFGF) was found to repress, rather than stimulate, SkA transcription in neonatal cardiac muscle cells. The proximal 200 nucleotides of a heterologous SkA promoter were sufficient for basal tissue-specific transcription, for induction by bFGF, and for inhibition by aFGF. Thus, both positive and negative regulation by peptide growth factors can be localized to the proximal SkA promoter. Full promoter activity required each of three CC[A/T]6GG motifs similar to the serum response element (SRE) for activation of the c-fos proto-oncogene, as previously shown for SkA transcription in a skeletal muscle background. The most proximal SRE, SRE1, was sufficient in the absence of other SkA promoter sequences for efficient tissue-specific expression in cardiac myocytes (versus cardiac fibroblasts), and was stimulated by bFGF to the same extent as the full-length promoter and endogeneous gene. Despite its ability to repress the SkA promoter, aFGF had no significant effect on SRE1. Both FGFs up-regulated the canonical fos SRE, to a comparable degree. Thus, SRE1 can discriminate between signals generated in cardiac myocytes by bFGF and aFGF. In cardiac myocyte extracts, two predominant proteins contact SRE1: serum response factor (SRF) and a second protein, F-ACT-1. Thus, serum response factor and F-ACT-1 are candidate trans-acting factors for basal transcription of the SkA gene in cardiac muscle cells and for induction of SkA by bFGF and, potentially, other trophic signals.